1. Field of the Invention
The present invention relates to a method of transforming an N-value image to an M-value image.
2. Related Background Art
An adaptive multi-value coding method has been hitherto known in which a distinction is made between a character portion and an (non-character) image portion of the original image so as to perform processes (types of processing) respectively suitable for the various portions based on the result of this distinction.
Such a conventional adaptive multi-value coding method employs a systematic dither method. The systematic dither method is employed because a threshold matrix array of the original binary image is fixed beforehand, so that the size of the threshold matrix can be estimated by performing pattern matching with the binary image.
In a binarizing method of a density preserving type for error diffusion or the like, the size of the threshold matrix is not fixed, nor is it estimated by pattern matching. Thus, image data which has been binarized by this binarizing method cannot be satisfactorily restored to the form of multi-value data.
On the other hand, conventional facsimile equipment has mainly performed binary monochromatic image communication. It seldom performs color image communication in view of the fact that no high performance color printer exists.
Thus, technology that may be encountered in relation to color image communication is not well understood. In actuality, almost no examination has been conducted concerning technology and circuitry used for transforming a binary image to a multi-value image.
In conventional processes for multi-value restoration, tables are utilized which hardware in actual use cannot hold; operation speed is not taken into account; and problems that may arise only when such a conventional process is employed with other image processing are not understood.
For example, when a binary color image is output by a high performance color printer where an image is received (on a receiving end), it must undergo various processes, such as a masking process adjusted to the developer of the color printer, and an MTF correcting process, before it can be output. Otherwise, the high performance color printer does not make use of its performance to the fullest extent. Also, even when a binary image undergoes image processes without being transformed to a multi-value image, it is not effectively processed and may be adversely affected. As a result, the resolution of characters deteriorates.
The assignee of the present invention, has proposed technology wherein it is determined whether or not a binarized image pattern is a half-tone image that has been processed by the dither method. A multi-value coding method is controlled based on the result of the determination.
There is room for improvement in such technology in that an image to be processed corresponds to a half-tone image, such as that processed for an error diffusion, which is not processed by a dither method.